1. Field of the Invention
The invention relates generally to a system for transferring electrical power from a source to a load. The invention relates more specifically to circuits and methods for efficiently converting the voltage and/or current parameters of electrical power from one form to another and for optionally correcting power factor error.
2. Description of the Related Art
Electrical power is often generated in one form and converted to another before being applied to an electrical load.
Form conversion can occur numerous times in the interim between when electrical energy is first produced by an electric generator and when it is finally delivered to a load and utilized for useful work. Nonefficient conversion wastes energy. A series chain of nonefficient power conversion steps multiplies the loss of any one conversion step within the chain. As such, it is desirable to maintain high conversion efficiency in each step of form conversion, particularly when a series chain of conversions takes place.
An excellent example of multiple form conversion is seen in the alternating current (AC) power distribution system of modern day electric utilities. An electromagnetic generator at a generating station converts energy from a waterfall or other energy source into an alternating electric current. A first voltage level is maintained at the generating point in order to meet specific requirements of the generating equipment and/or site.
Typically, a step-up transformer is provided at the electric generating station to transform the voltage/current parameters of the generator output from a form having a relatively low voltage and relatively high current to a form having a relatively high voltage and low current. The high-voltage power is then transferred over long-distance utility lines to a local distribution station.
A step-down transformer is provided at the local station for converting the high-voltage form of energy to a low-voltage form. In the United States, homes and small businesses typically receive their electrical power in the form of 60 Hz 120 volt.sub.RMS AC signals. (RMS represents "root mean square" which is a common measurement factor used for sinusoidal and non-sinusoidal signals.)
The instantaneous power present at the electric generating station, or anywhere else along the energy distribution network, is defined as instantaneous voltage (measured in volts) multiplied by the instantaneous current (measured in amperes). EQU P(t)=V(t).multidot.I(t). (Eq. 0.1)
One hundred watts of electric power can come in many forms, including but not limited to, a DC (direct current) signal having a voltage of 10 V and a current magnitude of 10 amperes, or one volt and 100 amperes, or 100 volts and 1 ampere. One hundred watts of electric power can also come in the form of an AC (alternating current) sinusoidal signal having a frequency of 60 cycles per second (60 Hz), a voltage of 110 V.sub.RMS and a current value of approximately 0.91 A.sub.RMS.
The term "power form" is repeatedly used herein to refer, in a broad sense, to the specific magnitudes of voltage V(t), and current I(t), of an electrical signal and to the way those magnitudes change over time and in relation to one another. The term "power form" applies to both AC and DC forms of electric power. In the case of AC electric power, the power factor (average power divided by V.sub.RMS .multidot.I.sub.RMS) and/or the phase correlation between voltage and current is included under the umbrella phrase, "power form." The symbol "Px" will be used to represent power form. The symbol "Pf" will be used to represent power factor.
In some instances, the power form (e.g., Px=120 V AC, 60 Hz single-phase) that is delivered by the utility company to a residential outlet can be applied directly to a working load. Examples of such working loads include single-phase AC electric motors which have been designed to operate efficiently under this power form, standard incandescent light bulbs and electric toasters.
In many other instances, however, the working load can not conform to the utility power form (e.g., 60 Hz 120 VAC) because the working load inherently requires a different power form. Conversion from the Px=60 Hz, 120 VAC power form to another power form has to be carried out in such cases. Examples of nonconforming working loads include but are not limited to: electronic circuits which operate at a standardized DC level (e.g., 5 V.sub.DC for TTL logic circuits); medical equipment which needs to be operated at low, isolated voltages for safety reasons; electro-optical devices, and electro-chemical work loads whose operating voltages or current magnitudes are dictated by physics and chemistry.
For purposes of illustration, we will assume the working load is a 12 volt lead-acid battery which is to be charged with electric energy delivered from the electric utility company to a residential outlet. This is an example of an electro-chemical work load whose operating voltage is dictated by the chemistry of the lead acid cell. Lead-acid batteries are preferably operated at or near integral multiples (e.g, 1-times, 2-times, 6-times) of the basic cell voltage: 2 volts. The utility power form (120 VAC, 60 Hz) is therefore preferably converted to another power form (e.g. 12 V DC) that is more suited for charging the lead-acid battery.
Many relatively complex designs are available in the arts of power converters and battery chargers for performing conversion from 120 VAC, 60 Hz to 12 V DC. See for example. U.S. Pat. No. 5,013,992 issued to Eavenson et al., May 7, 1991 "MICROPROCESSOR CONTROLLED BATTERY CHARGER".
Conventional converter designs suffer from a number of drawbacks including: power inefficiency, excessive cost, large weight and/or size and non-compatibility with sources and loads having/requiring power forms other than the ones which the converter was specifically designed for.
For purposes of further explanation we will assume a simple AC-to-DC, 120 V-to-12 V converter consisting of a step-down transformer connected to an AC outlet, a half-wave rectifier connected to the low voltage side of the transformer, a filter capacitor coupled across the output of the half-wave rectifier and a voltage-dropping resistor interposed between the filter capacitor and the working load (the 12 V chargeable battery).
Assume in this example that the step-down transformer converts the 120 VAC, 60 Hz utility voltage into a 60 Hz sinusoidal signal having a peak-to-peak magnitude of 40 volts. The half-wave rectifier charges the filter capacitor to a peak 20 volt DC level once every cycle; provided that the voltage of the capacitor drops below the 20 V peak level during the positive half of each such cycle. DC current flows from the filter capacitor through the voltage-dropping resistor into the 12 V battery. The voltage-dropping resistor absorbs the voltage difference between the 20 V peak across the capacitor and the 12 volts or less found across the battery under charge.
There are many inefficiencies in such a system. One major inefficiency comes from the fact that excess voltage is wasted away in the form of heat generated by the action of the voltage-dropping resistor.
Another inefficiency comes about because current does not flow through the half-wave rectifier when the 60 Hz waveform is in the negative half of its cycle. Even when the 60 Hz waveform enters the positive half of its cycle, current does not begin to flow through the half-wave rectifier until the rectifier input voltage exceeds the voltage then present across the filter capacitor. A large current spike develops at that time. Then, once the peak of the positive phase is reached, and the filter capacitor charges to peak, current flow abruptly stops because the rectifier becomes reverse biased. So current again ceases to flow.
This is not an efficient way to transfer power from the utility generator to the working load. Because instantaneous power is defined as P(t)=V(t).multidot.I(t), no instantaneous power flows through the rectifier during the entire negative half of each 60 Hz cycle, no power flows through the rectifier while the 60 Hz input voltage begins its climb from zero to the capacitor level during the start of the positive half cycle, and no power flows through while the 60 Hz input voltage drops from peak to zero during the end of the positive half cycle. As a result, power transfer is concentrated into a short time period in which a large burst of energy has to pass through the rectifier. This disadvantageously strains the rectifier.
Also, the power factor seen looking from the utility side into the half-wave rectifier is much less than 100%. This is undesirable. An "ideal," purely-resistive load would exhibit a 100% power factor. Capacitive or inductive loads have lower power factors and the phase relation between voltage and current is lagging or leading depending on whether the load is capacitive or inductive. The low power factor creates a disadvantageous energy loss along the utility lines.
In addition to these drawbacks, the hypothetical half-wave rectified converter that is being considered here has limited utility. It can not be used to charge a 24 volt lead-acid battery because the output of its step-down transformer is fixed to 20 volts peak. (The utility voltage is assumed to be fixed to 120 VAC here.)
If the load is changed to a 6 volt battery instead of a 12 V battery, the efficiency of our hypothetical converter would drop off significantly. Thus it would not be advisable to use the converter with load that requires a power form very different from that which the converter was designed to service.
Problems would also be encountered if the input voltage, and/or frequency, and/or waveform were to be changed. A converter that is designed to operate from a 120 VAC 60 Hz US residential source, for example, would not necessarily operate properly if driven by a 400 Hz airborne source or a 220 V 50 Hz European source.
Those skilled in the art will recognize that many techniques have been developed in an attempt to overcome some of the above-mentioned drawbacks. Unfortunately, many of the techniques are complex, costly and still fail to overcome all the drawbacks.